1. Field of the Invention
This invention relates to the thermal protection of particulate filters, and more particularly relates to thermally protecting particulate filters through physical heat dispersion.
2. Description of the Related Art
Recent emissions regulations have required the use of particulate filters in many internal combustion engine applications. The particulate filters capture soot from the exhaust stream, and the captured soot is later oxidized and vented to the atmosphere as carbon dioxide or carbon monoxide. If excessive soot is allowed to build up on the soot filter, the rate of soot oxidation can generate large amounts of heat in a short period of time and cause temperature spikes and large temperature gradients. Temperature spikes can melt the particulate filter, while temperature gradients can cause cracking or other stress related failures.
Conventional technologies try to control temperature spikes by controlling soot oxidation rates. This is usually achieved by limiting the amount of soot that accumulates on the particulate filter before oxidizing the soot in a regeneration event. While some of the conventional technologies have markedly improved the reliability and service life of particulate filters, there are still some problems that conventional technologies have not solved. Estimating the amount of soot on a particulate filter is an exceedingly difficult task, and direct measurements of soot are not currently available. Analogs for measuring soot, such as measuring the pressure drop across the particulate filter, have limitations in many applications due to low exhaust flow rates and uneven soot distribution within the particulate filter. Even when an overall soot estimate in a particulate filter is available and accurate, localized soot concentrations can occur within the particulate filter.
Further, conventional technologies can still be subject to failures even when the technology is working perfectly. For example, the soot estimation may be functional, but the application may operate in a manner that prevents regeneration of the particulate filter for extended periods of time due to insufficient temperature generation. Further, a sensor or other failure may prevent the soot estimator from generating a soot estimate even if the soot estimator is functioning properly. In these and similar cases, a high temperature regeneration of the particulate filter can cause a large temperature spike or gradient and damage the particulate filter.
Within the wall flow particulate filter where exhaust flow is forced through the substrate wall of the particulate filter, the highest nominal concentrations of soot occur at the end of soot channels. A zone near the exit of the particulate filter tends to experience the highest soot loadings, and therefore the highest soot oxidation exotherms. The center of the particulate filter tends to experience the highest temperatures because the surrounding particulate filter insulates the center and slows heat transfer to the environment.
These effects combine such that the vast majority of temperature related failures in particulate filters occur in a centralized zone near the downstream side of the particulate filter. The exact size and shape of this zone, which may be called the high risk portion of the particulate filter, can vary according to the particular application, the type and amount of soot deposition within the particulate filter, the availability and type of regenerations experienced by the soot filter, and the ambient environment and insulation of the particulate filter. Those of skill in the art learn the high risk portions of a given application through field experience and standard analysis of failed particulate filters.
The physical protections available for particulate filters in the current art have shortcomings. Two primary methods are currently used to protect the particulate filter. The first is to manufacture the particulate filter completely out of a material that can withstand extreme temperatures and temperature gradients. The second is to manufacture the particulate filter substrate such that the particulate filter has a large overall thermal mass or heat capacity.
A common example of the first method is to manufacture the entire particulate filter out of silicon carbide. While this is a robust material, silicon carbide has some undesirable qualities. For example, silicon carbide has a high specific heat capacity, reducing the ability to reach a soot oxidation temperature, and silicon carbide cannot accept as high of washcoat loadings as some competing materials. Further, even a silicon carbide particulate filter can fail under high temperatures and thermal gradients. Rather than over-designing the entire particulate filter, it would be desirable to focus improvements on the high risk portion of the particulate filter.
A common example of the second method is to manufacture the entire particulate filter with thick channel walls and thereby supply more filter material to be heated. This method has several drawbacks, however. First, the combination of thicker substrate walls and smaller channel areas provides for a greater pressure drop than filters designed to meet filtering criteria rather than temperature resistance criteria. Second, the portion of the particulate filter that is not at high risk also must have a high heat capacity under such a design, since the substrate wall thickness must be experienced throughout the filter. Since all of the thermal mass of the filter must be heated during a regeneration event, it is less fuel efficient to perform a regeneration in such a filter. Further, applications which can only marginally achieve a regeneration under normal circumstances may not be able to achieve a regeneration in an application where the entire filter has a high heat capacity.
From the foregoing discussion, it should be apparent that a need exists for an apparatus, system, and method that provides for dispersing heat within a particulate filter such that temperature spikes and gradients are avoided. Beneficially, such an apparatus, system, and method would disperse heat through time, space, molecular energy storage, and across aftertreatment components.